Accurately measuring and controlling temperature is very important throughout industry. Temperature sensors are at the heart of all temperature control systems, and usually use thermistors as a sensing element since they are cheap and convenient.
A thermistor is a semiconductor with suitable resistivity and temperature coefficient, which is fabricated by mixing oxides of Co, Mn, Fe, Ni, Ti, etc. and then sintering the mixture. The conductivity of a thermistor changes with temperature. Unlike general metals, a thermistor has a negative temperature coefficient (NTC), meaning its resistance decreases as temperature increases. NTC thermistors are widely used in devices that sense temperature.
FIG. 1 is a circuit diagram of a conventional alarm apparatus using a thermistor.
Referring to FIG. 1, the conventional alarm apparatus includes a sensor unit (denoted by a dotted-line rectangle) and an alarm-signaling unit. The sensor unit includes a thermistor 10, an amplifier 20, and a transistor 30. The alarm-signaling unit includes a buzzer 50 and a light-emitting diode 60. The sensor unit and the alarm-signaling unit are connected through a relay switch 40.
As the resistance of the thermistor 10 changes with temperature, the voltage at a negative input terminal of the amplifier 20 changes and thus the output voltage of the amplifier 20 changes. The output voltage of the amplifier 20 is applied to a base terminal of the transistor 30. The transistor 30 is turned on when the applied output voltage reaches a specific voltage. Accordingly, the relay switch 40 is also turned on, to operate the buzzer 50 and the light-emitting diode 60.
The conventional alarm apparatus using a thermistor is widely used, but is complex because it includes an amplifier, a transistor, and a plurality of resistors, as illustrated in FIG. 1. In addition, the temperature threshold of the conventional alarm apparatus is fixed at a specific temperature, specified by the resistance change rate of the thermistor.
Another example of a temperature sensor is a bimetal temperature sensor. The bimetal temperature sensor is also inexpensive and widely used, but has a very wide range of temperature thresholds, and thus is difficult to use for accurately sensing a desired temperature.
A further example of a temperature sensor is a ceramic temperature sensor using vanadium dioxide (VO2). The vanadium dioxide undergoes a structural phase transition from a monoclinic system to a tetragonal system at a critical temperature of about 68° C., and thus changes in electrical resistance. However, this critical temperature is not adjustable, and the ceramic temperature sensor is easily broken down by a large current.